Thermal relays conventionally used to control circuits typically include a laminated bimetallic element, the two materials of which have different thermal coefficients of expansion, so that the bimetallic element moves from one position to another in response to an increase or decrease in ambient temperature. A contact carried by the bimetallic element is thereby moved towards or away from a fixed contact in response to ambient temperature changes to make or break a circuit. The movement is gradual, causing engagement and disengagement between the contacts to be effected too slowly. Contact wear is accelerated and the switch fails prematurely.
To increase the velocity of the movable contact, bimetallic elements have been formed with cutout sections or with curved cross sections that provide a "snap" action. Such modifications increase the cost of the relay and subject the element to Joule heating, which can adversely alter the characteristics of the bimetal, and hence the accuracy and reliability of the relay. Relay constructions wherein relative movement between the contacts is due in part to temperature characteristics of a magnetic material, such as those taught by U.S. Pat. Nos. 2,951,927 and 3,287,541, provide contact velocities too slow to prevent contact wear and premature switch failure. For these reasons, temperature-sensitive relays of the type described have resulted in higher purchase and maintenance costs than are considered to be commercially acceptable.